Production of cellular concrete blocks



Patented Apr. 14, 1953 [iii/WNW h UNITED STATES PATENT OFFICE PeterEdward Starnes, Boreham Wood, England,

assignor to John Laing & Son Limited, London,

England, a British company Application October 20, 1948, Serial No.55,647. In Great Britain October 22,

No Drawing.

2 Claims. (Cl. 106-87) The present invention relates to the productionof cellular concrete for use in the manufacture of blocks, slabs, beamsand the like and has for its object an improved composition and processof manufacture.

The manufacture of any light-weight cellular composition suitable forsatisfactory use as a structural material in building presents twodistinct problems.

(a) The problem of producing a paste Or slurry of suitable siliceoussubstances capable of hardening into a strong, durable materialpossessing a minimum of microscopic capillaries and pores and exhibitingthe smallest possible changes of dimensions on wetting and drying.

(b) The problem of inducing a uniform cellular, or foam-like, structurein the said paste or slurry, which will remain substantially unchangedduring the subsequent processing and curing of the composition, so thata strong, light and relatively impermeable cellular material willresult. In this connection it has been found that certaincharacteristics are desirable in the cellular structure, while othersare equally undesirable. It is desirable that the structure shallconsist of uniformly sized, spherical cells and that there shall be aminimum of interconnection between neighbouring cells. This isdesirable, not only in order to attain the maximum strength for a givendensity, but also in order to restrict the circulation of air and watervapour throughout the composition, and hence to ensure the lowestpossible thermal conductivity. It is undesirable that the cells shouldbe distorted out of the spherical shape into ellipsoids so as to producethe appearance of an horizontally laminated structure when a verticalsection of the material is viewed under the microscope. This fault,which is common in chemically expanded light-weight concrete results ina difference in the compressive strengths of the product when tested intwo mutually perpendicular directions. It is also undesirable that largebubbles or voids be formed in the material either by the coalescence ofsmall bubbles or by the accidental entrainment of air during the mixingor pouring of the composition.

In practice I have found that these two prob-" lems are actuallyinter-related in as much as attempts to reduce the movements on wettingand drying of the material by the incorporation of a quantity of coarsersand as described below will usually result in the formation of a moreor less laminated structure. together with a variation in the size ofthe cells throughout the body of the composition when the latter ispoured into moulds to form units suitable for use in building.

The present invention relates to both these problems. In the first placeit has as an object an improved composition which when aerated into acellular structure and suitably cured as described herein, will form alight-weight cellular material of great strength and durability, whichexhibits relatively small movements on wetting and drying. In the secondplace it has for an object an improvement to the known method ofaerating such a composition by the production of a gas within the massby the reaction of a finely divided metal and a weak solution of alkaliwhereby the uniformity of the cellular structure may be controlled andthe tendency for the production of a laminated struc ture eliminated.

The restrictionaof-the drying shrinkage and movement on wetting of anylight -wight concrete composition within an acceptable limit is ofprimary importance if it is intended that the material should besuitable for exterior exposure in a normally constructed building. It isunfortunate in this respect that the circumstances which lead to thedevelopment of a high strength in a siliceous cement paste also tend toresult in an increase in the movements on wetting and drying of thehardened product. This is particularly true in light-weight compositionsof the cemen o a sma uantit of ver n trass or other natural pozzolanacan result when u HIT-I $231221. This is attributed to the reduction ine uncombined lime content and the formation of a proportion of silicatesof lower basicity such as mono-calcium silicate hydrate in the pasteinstead of the dicalcium silicate hydrate which is normally theprincipal compound formed during the hardening of a Portland cementpaste. It is also known that curing a Portland cement paste at elevatedtemperatures in the presence of saturated steam will, provided thetemperature is suificiently high, result in a great reduction of theshrinkage and expansion of the hardened paste on drying and wetting.This is likewise to be attributed to the increased tendency for theformation of a crystalline silicate hydrate of low basicity. It is to beexpected that a combination of both these methods, i. e. theincorporation of a proportion of finely divided active silica and theuse of high pressure steam curin would result in an even greatertendency for the formation of the monocalcium silicate hydrate, and thishas been shown to be the case. By the choice of an appropriate source offinely divided active silica it has been found possible greatly tocheapen the cost of a suitable low shrinkage cementing material of greatdurability when compared with the cost of a normal Portland cement pasteof equivalent strength and durability.

At the same time the rate of strength gain is much increased and hencethe curing time reduced.

Further by the incorporation of a proportion of suitably graded sandwith the cementing mixture I have found it possible to produce a'hardened paste of great strength, durability and further improvedshrinkage characteristics. This has been made possible by the discoverythat certain chemical agents herein described are capable of exerting astrong stabilising influence on the cellular structure of the uncuredpaste so as to permit of the admixture of a proportion of coarseraggregate which would otherwise interfere with the development of auniform structure and in particular result in the production ofundesirable laminations.

It has previously been recorded that some samples of pulverised fuel ashexhibited slight pozzolanic activity when admixed with Portland cementin the preparation of ordinary dense concrete, and this is to beexpected when the chemical nature and state of division of the materialare considered.

I have studied the nature and reactivity of various samples of p verise'fuel ash, an ave a ren samp es vary greatly in their chemicalreactivity. In general, it was possible to distinguish two main groups.

(a) Ash collected from the furnace gases by 'means of cyclone typeseparators. This type was always less reactive than type (b) -(b) Ashcollected from the furnace gases by means of electrostaticprecipitators. This type was always more reactive than type (a).

On microscopic examination of the samples it was found that all those oftype (b) were characterised by containing a reasonable proportion ofvery fine particles (say less than microns in diameter). Ashes of type(a) contained very 5 much fewer particles smaller than about 15 example,below are given the specific surfaces in square centimetres per grammeof particles of different sizes having a specific gravity of 2.2

assuming all the particles to be smooth and spherical.

Size of Particles From this it can be seen how important is the partplayed by the particles smaller than 15 and particularly by ultra fineparticles less than about 5 in diameter.

The problem of selection of a suitable ash is to some extent complicatedby the fact that all samples of pulverised fuel ash contain a quantityof unburnt carbon, usually as irregular particles of coal or coke about50-100 microns in size. If present in any considerable proportion thiscarbon exerts a deleterious effect upon the product. This occurs in twoways. Firstly, the carbon may be regarded as an inactive adulterantwhich not only fails to contribute to the development of strength in thematerial, but may also by virtue of its own porosity and absorptionincrease the porosity and movement on wetting and drying of the product.Secondly, it may so increase the quantity of water necessary to producea suitable paste or slurry as further to detract from the quality of theproduct both as regards strength and impermeability to water.Fortunately I have found that ashes with carbon contents in the rangenormally experienced (say 5-l5%) can be used quite satisfactorily. It iswise however to select a source of ash so that the carbon content doesnot exceed 10-12% if the maximum economy of cement is to be affected inthe proportioning of the mix.

I have found that pulverised fuel ash when selected according to theabove principles and mixed together with Portland cement and a suitablygraded siliceous sand and water in the correct proportions and aeratedaccording to the method described below will on hardening give rise to aparticularly strong and durable lightweight composition of lowpermeability. If the full benefit of the composition is to be realisedit is desirable to cure the material in high pressure steam at atemperature exceeding l'75 C. This applies particularly when the productis desired to have a density less than 60-70 lbs/cu. ft. and when it isnecessary to achieve the minimum changes in dimensions on wetting anddrying.

With regard to the grading of the sand I have found that the use ofstabilising agents described below permits of a considerable latitude.The choice is of course influenced by the density which it is desired toachieve in the final product. In general the sand should have a highsilica content, be clean, free from contamination by clay or organicmatter and have a grading in the ran e indicated below. A sand at thecoarser end of the range is suitable for the denser products, while asand of the finer type is suitable for the light-weight products andparticularly for products hardened in high pressure steam.

Range of suitable gradings Percentage B. S. Sieve No. Passing tion ofthe coarser material is present. In particular two faults are liable tobe developed in this case. The individual bubbles or cells becomeflattened into an ellipsoidal shape due to the tendency for the aeratedslurry to settle slightly in the mould. Also it is common to find thatthe density of the material increases from the top to the bottom of themould due to the compression of the lower bubbles. Both these faultsbecome more serious as the depth of the mould is increased. and in factset a practical limit to the depth of the mould which can be used in themanufacture of units from a chemically expanded composition of the typewith which this application is concerned. I have discovered that theaddition of a very small proportion of certain water soluble compoundsof high molecular weight to the slurry greatly increases the stabilityof the cellular structure in this respect and permits the incorporationof a higher percentage of coarse materials while at the same timeallowing the use of moulds considerably deeper than was hithertopossible under these circumstances.

In particular two water soluble cellulose compounds known respectivelyas ethylmethyl cellulose and sodium carboxymethyl cellulose are veryvaluable. Using an addition of a small quantity of a solution of one ofthese compounds together with a suitable wetting agent both the size andregularity of the pore structure can be controlled. In general thehigher the proportion of the soluble cellulose compound added the finerwill be the pore structure.

The precise mechanism by which these agents act is not known. Twoproperties which they possess appear to be of prime importance however.

(1) They exhibit the tendency to concentrate as an absorbed layer at aninterface.

(2) Their solutions are characterized by exhibiting a very highviscosity even at very low concentrations.

It is tentatively assumed that the stabilisation observed is primarilydependent upon the result of the combination of these two properties.That is to say there is produced a concentration of the agent in thewall of a bubble or cell as soon as it is formed which greatly increasesthe .interfacial viscosity and stiifens and strengthens the system. Itwould not appear to be irrelevant in this connection that the compoundsherein described as being most successful stabilizers are characterisedby the fact that they possess polar groups distributed along the lengthof the molecule, so that one would expect the absorbed molecules to liemore or less parallel to the interface instead of at right angles to itas is the case with the common surface active materials of thehydrocarbon sulphonate type.

If this is the case one can postulate that the presence of a number ofthese long molecules lying more or lessin the surface could behave tosome extent as a reinforcement to the fabric of the bubble or cell in amanner not possible with the more simply dipolar surface activecompounds which become orientated perpendicular to the interface.

It is to be noted that there exist substances of similar character tothe agents named, which are also capable of exerting a stabilizingaction on a cellular mix in a like manner and the use of any such agentfor this purpose as described herein is included within the invention.For

this reason it is now proposed to define the nature of these compounds.

For the purposes of this invention a cell stabilising agent is definedas a water soluble organic compound of high molecular weight which issurface active and which forms aqueous solutions (true or colloidal)characterised by their abnormally high viscosity at low concentrationsof the solute or dispersed phase. structurally these compounds aregenerally distinguished by the possession of a number of polar orlyophilic groups distributed along the length of the main molecularaxis.

As examples the two soluble cellulose ethers referred to above may becited.

Example (A) .--Ethylmethyl cellulose A mixed ethyl methyl ether ofcellulose having generally a degree of substitution in the range of 0.4to 1.5 hydroxyl groups per glucose unit when produced commercially.

Example (B) .-Sodium carboxymethyl cellulose The sodium salt of acarboxymethyl acid of cellulose having generally a degree ofsubstitution in the range of 0.45 to 0.75 hydroxyl group per glucoseunit when produced commercially.

Not all wetting agents are equally suitable for use with thesestabilisers. I have found that wetting agents of the type which possesshydrophilic groups of the non-dissociating type to be satisfactory ingeneral. In particular a proprietary product known as Lissapol N whichis described in the literature as being the product of condensingethylene oxide with a substituted phenol or fatty. alcohol has proved tobe quite satisfactory and is used as a convenient standard in theexamples of suitable compositions which follow.

In my experiments I have used a finely divided type of aluminium powderas the aerating agent, together with a small quantity of caustic sodasolution. I have found that six factors are of prime importance in orderto ensure a constant degree of aeration and hence a constant density andstrength.

(l) The fineness of the aluminium powder.

(2) The quantity of the alkali used.

(3) The temperature of the mix.

(4) The consistency of the mix.

(5) The timing of the mixing cycle and particularly the length of timeelapsing between the addition of the aerating agents and the dischargingof the mix into the mould from the mixer.

(6) The nature and proportion of the stabilising agents used.

Factors (1) and (2) are inter-related in as much as the finer the stateof division of the aluminium powder the less is the quantity of alkalirequired. The quantity of alkali required must be determined byexperiment for each type of aluminium powder. It should be sufficient toensure that the total expansion required takes place in from 10-20minutes according to the density aimed at.

Aluminium powder is manufactured in a number of grades of fineness. Forthis purpose it should be at least as fine as that normally supplied foraluminium paint manufacture sometimes known as varnish grade" andpreferably finer. It should also havew tenLas-possiL I have found itadvantageous to use a mix temperature of approximately C. Usually thisrequires that the gauging water be added at a temperature of 60 to 70 C.

The consistency of the mix has a considerable effect upon the structureand strength of the product.

The ideal consistency is that of a rich creamy slurry, easily pourable,but not wet enough to bleed. The consistency is of course largelydetermined by the water content. Too little water results in unevenexpansion and the formation of large irregular cavities during thepouring. Too much lowers the strength and increases the absorption ofthe product while also giving rise to 15 a tendency for the mix to slumpin the mould. One of the secondary benefits derived from the use of thestabilising agents referred to is that the actual water content isrendered less critical in this respect. Owing to variations in thenatural moisture contents of the materials and also the free carboncontent of the pulverised fuel ash it has been found impracticable tocontrol the consistency accurately by gauging the water added to themix. In practice it has proved more satisfactory to add the water untilthe required consistency is attained as judged by eye or recorded by asuitable instrument.

With regard to the timing of the mixing cycle this, of course, dependsupon the size of the mix and the type of mixer. A few general directionsmay be given, however, and these together with a specific example shouldserve to demonstrate the important points. I have found that contrary togeneral practice, it is not advisable to add the aluminium powder withthe dry ingredients at the beginning of mixing period. If this is done Ihave found that reaction with the alkali derived from the cement, and insome cases from the pulverised fuel ash, occurs during the whole of themixing period. Not only is this wasteful of aluminium powder but also,owing to variations in the free alkali content of the cement andpulverised fuel ash, it leads to variations in the expansion produced. Ihave found it most advantageous to add the aluminium powder togetherwith the soluble cellulose compound and a suitable wetting agent between60 and 90 seconds before pouring the mix while the alkali should beadded from 30 to 60 seconds before pouring as a 30% solution of causticsoda.

As an example, I give below the mixing schedule for a mix of 10-15 cu.ft. mixed in an open pan mixer for material intended to have a densityof approximately 50 lbs/cu. ft.

Time in Seconds Schedule With regard to the quantity of stabilisingagent used, fairly wide limits can be set but the use of too high aproportion results in the inhibition of the aeration of the mix to someextent so that the final product is denser than desired. 7 Suitablequantities chosen in relation to the desired pore size, coarseness ofsand and so on are determined by experiment. It is important to notethat different samples of very similar chemical composition may vary intheir ability to stabilise the mix. It appears that standardisation bythe criterion of the viscosity of a dilute (say 1% of solution) of theagent is possible. Alternatively it is not difficult to make slightalterations to the proportions to adjust for variations from batch tobatch in the activity of the agent as manufactured.

Having discussed the general nature of the present invention it is nowproposed to specify in greater detail the range of compositions andprocesses covered with several examples.

The present invention has for its object an improved composition and amethod of manufacture thereof into a light-weight cellular concretecomposition in which the known pozzolanic properties of pulverised fuelash are utilised, the necessity for grinding the aggregate is eliminatedand the time of curing of the concrete is greatly reduced, while theproduct is characterised by its high strength, durability, lowabsorption and relatively small changes in dimension on wetting anddrying and also its uniformity of structure.-

The improved composition consists of a mix.- ture 91 Portland cement,pulwusmr fly ash a suitably graded s1 icegps sang and a miguLtitoflemfinelmdixideialuminium powder toget l rtion of a solut.oncontaining a suitable water soluble stabilising compounfanfi suitablewetting agent. a solution of caustic soda and water. The improved methodof manufacture thereof is concerned with the procedure to be adopted inthe mixing of the various ingredients, the pouring of the mixture intomoulds and the curing of the composition.

The following describes in detail the preparation and manufacture of alightweight composition having a density of about 55 lbs/cu. ft. Thedescription relates to a mix of approximately 15 cu: ft. of expandedconcrete composition mixed in a revolving pan mixer.

Example I The proportions given in the table are such as to produceapproximately 15 cu. ft. of the expanded material.

Aluminium Powder v. 9 30% Caustic Soda Solution 250-700 mls. accordingto reactivity of aluminium powder.

200 mls. according to 6% Solution of Sodium Garboxymcthyl Cellulose.size of pore desired an activit of sam l 10% Solution of Non-ionicWetting 3501'n1s. y p 0 Agent Lissapol N."

1 Grading of sand:

Percent B. S. Sieve No. Passing o ded as a suspension 111 a 1 ettingagen biliser. Mixing is continued until the aluminium powder iscompletely dispersed in the mix (about 30 seconds. The measured volumeof 30% caustic soda solution is now added and mixing continued foranother 60 seconds). By the end of this time if the concentration ofalkali has been correctly adjusted the evolution of gas will beproceeding vigorously within the mix. The mix is then ready for pouring.It is poured into an oiled mould which has preferably been preheated tothe temperature of the mix. Alternatively the sides of the mould may bemade of wood or some other material which is a poor conductor of heat.The quantity of slurry poured into the mould is adjusted so that whenthe rising is complete there is a slight excess of material over the topof the mould.

The mould and the contents are allowed to stand in a humid atmosphere atroom temperature for 3 hours and the excess material is out off. Thesides of the mould can then be removed and the material is ready forcutting into the required sections. Curing may then be commenced, eitherin moist air, low pressure steam or high pressure steam. The mixdescribed above is designed to be cured in saturated steam at a maximumpressure of 160 lbs/sq. in.

At this pressure a suitable curing cycle has been found to be 3 hoursfor development of pressure, 7 hours at full pressure and 2 hoursdepression to atmospheric pressure. With these conditions of curing thefollowing strengths will normally be obtained.

Lbs/sq. in.

Compressive strength wet 700-800 Compressive strength dry 950-1050Modulus of rupture dry 330-350 Example 11 Light-weight material ofapproximate density '15 lbs/cu. ft. produced with low pressure steam ormoist air curing and suitable for manufacture into internal partitionblocks. Strength approximately 600-800 lbs/sq. in. according to methodof curing.

Ingredients Proportions Portland Cement 1 part. Pulverised Fuel Ash 1.2parts. San 2.8 parts. Aluminium Powder 0.0015 part. 30% Caustic SodaSolution 1.5-3.5 mls. per lb. of

cement. 0.5-1.0 mls. per lb. of

% Solution of Sodium Carboxymethyl Celiul cemen 2.0 mls. per 1b. ofcenent.

ose. Solution of N on-ionic Wetting Agent issapol N".

Example III Light-weight material of approximate density 65 lbs/cu. ft.produced with low pressure or high pressure steam curing. Strength600-900 lbs/sq. in. according to method of curing.

5 Solution of Sodium Carbo methyl lellul Xy cement. 0.3-0.8 mls. per lb.of ose. nt 10% Solution of N on-ionic Wetting Agent Lissapol N.

ceme 1.5 mls. per lb. of cement.

Example IV Light-weight material of approximate density 45 lbs/cu. ft.suitable for use where a structural material of high insulation valueand low shrinkage is required. Curing should be in high pressure steamat lbs/sq. in. to ensure minimum changes in dimensions on wetting anddrying.

Ingredients Proportions Portland Cement 1 part. Pulverised Fuel Ash 1.2parts. Sand 0.8 part. Aluminium Powder 0. part.

1.2-3 5 mls. per lb of 30% Caustic Soda Solution 5% Solution of SodiumCarboxymethyi Cellulose. 10% Solution of Non-ionic Wetting Agent ceme1.5 mls. per lb. of cement. Lissapol N".

The above examples serve to illustrate the general range of compositionsincluded in this invention, but of course the examples given must not beheld to limit the invention as there is a very large number of suitablecombinations of the basic ingredients which will provide compositionsfor manufacture into materials of various densities, strengths, thermalconductivities and shrinkage characteristics and suited to a variety ofcuring procedures. The choice of a particular composition for a givenpurpose is governed not only by technical considerations but also by theeconomic factors involved, and it should be particularly noted that oneconsiderable advantage offered by the successful utilisation of a wastematerial such as pulverised fuel ash in the production of a high qualitylightweight material as herein described lies in the reduction in costof manufacture effected.

In fact it is true to say that using the compositions and methodsspecified it is possible to manufacture a material of improved qualitiesat a cost which is lower than that for conventional light-weightPortland cement:sand or Portland cementzpozzolanazsand mixes whether thepewiana be either a natural trass or other material specially ground oran artificial product specially prepared by heating and grinding somenatural siliceous material.

As an approximate range of the useful and economic compositions thefollowing proportions may be given:

Portland cement, 1 part Pulverised fuel ash, 0.3-1.5 parts Sand, 0.5-4.7parts Powdered aluminium, .001-0.5 part 1.2 to 3.5 mls. per pound ofcement of a 30% caustic soda solution with a very small proportion ofsodium carboxy methyl cellulose or of ethyl methyl cellulose and of anon-ionic wetting agent and water.

I claim:

1. Method of producing lightweight concrete consisting in mixing about 1part of each by weight of Portland cement and ash from th combustion ofpulverised bituminous fuel in furnaces and recovered from the combustiongases by separators with 0.5 to 4.7 parts of sand, adding to the mixturewater at a temperature of about 60 to 70 C. in sufficient quantity toform and forming therewith a creamy slurry, adding aluminum powder tothe slurry as a dispersion in a solution containing about 0.3 to 1 ml.per pound of cement of a 5% solution of a water soluble stabilisingagent of high molecular weight which is surface active and forms anaqueous solution of high viscosity selected from the group consisting ofsodium carboxy methyl cellulose and ethyl methyl cellulose and about 1.5to 2 mls. per pound of cement of a 10% solution of a wetting agentpossessing hydrophilic groups of the non-dissociating type, mixing thedispersion with the slurry to produce a thorough dispersion of thealuminum powder, and then adding alkali shortly before discharging themixture into a mould.

2. Method of producing lightweight concrete consisting in mixing about 1part each by weight of Portland cement and ash from the combustion ofpulverised bituminous fuel in furnaces and recovered from the combustiongases by separators with 0.5 to 4.7 parts of sand, adding to the mixturewater at a temperature of about 60 to 70 C. in suflicient quantity toform and forming therewith a creamy slurry, adding aluminum powder tothe slurry as a dispersion in a solution containing about 0.3 to 1 ml.per pound of cement of a 5% solution of a water soluble stabilisingagent of high molecular weight which is surface active and forms anaqueous solution of high viscosity selected from the group consisting ofsodium carboxy methyl cellulose and ethyl methyl cellulose and about 1.5to 2 mls. per pound of cement of a 10% solution of a wetting agentpossessing hydrophilic groups of the non-dissociating type, mixing thedispersion with the slurry to produce a thorough dispersion of thealuminum powder, adding alkali shortly before discharging the mixtureinto a mould, allowing the mixture in its mould to stand in a humidatmosphere at room temperature for a number of hours and then ouring themix by means of saturated steam.

PETER EDWARD STARNES.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,954,117 Caldwell Apr. 10, 1934 2,205,735 Scherer June 25,1940 2,432,971 Ruthman Dec. 16, 1947 2,476,306 King July 19, 19492,534,915 King Dec. 19, 1950 2,560,871 Johnson July 17, 1951 FOREIGNPATENTS Number Country Date 243,308 Great Britain Dec. 9, 1926 480,681Great Britain of 1938

1. METHOD OF PRODUCING LIGHTWEIGHT CONCRETE CONSISTING IN MIXING ABOUT 1PART OF EACH BY WEIGHT OF PORTLAND CEMENT AND ASH FROM THE COMBUSTION OFPULVERISED BITUMINOUS FUEL IN FURNACES AND RECOVERED FROM THE COMBUSTIONGASES BY SEPARATOR WITH 0.5 TO 4.7 PARTS OF SAND, ADDING TO THE MIXTUREWATER AT A TEMPERATURE OF ABOUT 60* TO 70* C. IN SUFFICIENT QUANTITY TOFORM AND FORMING THEREWITH A CREAMY SLURRY, ADDING ALUMINUM POWDER TOTHE SLURRY AS A DISPERSION IN A SOLUTION CONTAINING ABOUT 0.3 TO 1 ML.PER POUND OF CEMENT OF A 5% SOLUTION OF A WATER SOLUBLE STABLISHINGAGENT OF HIGH MOLECULAR WEIGHT WHICH IS SURFACE ACTIVE AND FORMS ANAQUEOUS SOLUTION OF HIGH VISCOSITY SELECTED FROM THE GROUP CONSISTING OFSODIUM CARBOXY METHYL CELLULOSE ETHYL METHYL CELLULOSE AND ABOUT 1.5 TO2 MLS. PER POUND OF CEMENT OF A 10% SOLUTION OF A WETTING AGENTPOSSESSING HYDROPHILIC GROUPS OF THE NON-DISSOCIATING TYPE, MIXING THEDISPERSION WITH THE SLURRY TO PRODUCE A THOROUGH DISPERSION OF THEALUMINUM POWDER, AND THEN ADDING ALKALI SHORTLY BEFORE DISCHARGING THEMIXTURE INTO A MOULD.